1. Field of the Invention
The present invention generally relates to the art of surface finishing of aluminum. More particularly, it is concerned with a method of anodizing aluminum or aluminum alloys to provide the surface thereof with a porcelain-like texture, or a grainy touch having a timber, leather or sand texture of aesthetic value.
2. Description of the Prior Art
The method of finishing the surface of aluminum which involves the art of forming a protective film layer of .gamma.-Al.sub.2 O.sub.3 over a base metal by anodizing it in solutions including various electrolytes has been known in the art since the endof the 19th century.
In Japan, anodized aluminum products obtained by the use of an electrolyte of oxalic acid have been very popular since the early 1930's to the extent that the trade name "Almite" has become an accepted pronominal designation for the entire spectrum of anodized aluminum products.
Moreover, a number of papers have been published and many patents have been issued on or relating to the art of aluminum anodizing. These various references disclose the composition of the electrolyte solution, the conditions of electrolysis, the species or properties of the base metals, the pre- and post- treatment thereof for elaborately modifying the tone and color, the optical reflectance, the hardness and the flexibility of the film layer thus formed.
Among the prior art methods customarily known as "direct coloring", which are devised for depositing an appropriate metal salt in the film layer simultaneously with the formation of the layer by means of incorporating the metal salt in the electrolyte solutionused in the anodizing operation, the most famous one is the "Ematal verfahren", disclosed in the specification of German Pat. Nos. 655,100 and 672,268 issued to Schenk. According to these disclosures, the addition of salts of titanium, zirconium, thorium and the like in the conventional electrolyte solution containing oxalic acid, and the selection of direct current anodizing conditions of terminal voltage from 2-3 V to 120 V and a bath temperature ranging from 50.degree. to 70.degree. C., will enable the deposition of the metal salts, for instance, titanium oxide in the film layer to give the layer an opaque milky-white or porcelain-like appearance.
However, despite many efforts of the present inventor, experiments performed for confirming the stated advantages of the "Ematal verfahren" have failed to provide the preferred results of the expected oxide film having a white porcelain-like texture. In addition to the pure expense of the salts of titanium or zirconium, the "Ematal verfahren" procedure has thus been found to be unsatisfactory for commercialization or very difficult to perform as a routine operation, if not completely impossible.
As has been and is now generally accepted, white porcelains descended from the old age of China pottery patterned in the ancient traditions are highly evaluated. Among them, the tone and color of the pale-white texture of porcelains from the pottery kilns in the Southern District are regarded as being highly prized with keen envy by many enthusiasts and art collectors.